1. Field of the Invention
The present invention relates to an image pickup system.
2. Description of the Related Art
In a conventional digital still camera, an AF (auto focus) type such as the following has been realized.
In other words, a sensor for AF has been provided separately apart from an image pickup element. A typical example is a phase-difference AF which is used in a single-lens reflex camera. A merit of this type is that a problem of parallax is solved by using a light beam which has passed through a taking lens, and it is possible to detect a focal shift by a one-time defocus amount detection signal, and to focus the taking lens to an object, and it is possible to shorten time till focusing.
However, in the abovementioned type, it is necessary to dispose an optical path switching unit or an optical path dividing unit between the taking lens and the image pickup element, and furthermore, an optical system for AF is necessary between the sensor for AF and the optical path switching unit or the optical path dividing unit. This demerit leads to an increase in size from a point of (1) securing space for the optical path switching unit, and a point of (2) securing space for a separate sensor and the optical system for AF. Furthermore, (3) in a case of disposing the optical path switching unit, there is a time lag due to switching with a switching mechanism, and (4) in a case of disposing the optical path dividing unit, there is a loss of amount of light due to division of optical path.
Also, there is an AF type which uses an output of an image pickup element. A typical example is a contrast AF which has been used in a so-called compact digital camera or a video camera. In this type, the image pickup element and the AF sensor being integrated, there is a merit of space and a merit of lesser number of structural members.
However, in the contrast AF, there are demerits that it is necessary to compare contrast signals of plurality of times, and due to wobbling, it takes time till focusing. Furthermore, in video photography, blurring of image due to wobbling is also photographed, and an image quality is degraded. Larger the number of pixels, this tendency is more conspicuous.
Furthermore, in recent years, with respect to the single-lens reflex camera which includes an optical finder in which a quick-return mirror and a roof prism (Dachkantoprism) are used, an interchangeable lens camera which is premised on an electronic view finder which does not require the quick-return mirror and the roof prism, has been released, and a high image quality has been sought both in still photography and video photography. In this interchangeable lens camera, an AF which has the merits of both AF types mentioned above, and in which, the demerits of both the type are eliminated, has been sought. Moreover, even as an image pickup system in which, the taking lens is fixed to a camera body, a system with a short focusing time has been sought.